In the known interfacial process for preparing polycarbonates, solvents such as aromatic chlorohydrocarbons such as chlorobenzene and dichloromethane are used, the residual contents thereof in the end product being unwanted since they are disruptive in the polycarbonate. In order to remove these volatile constituents, the devolatilizing extruder has to be operated at relatively high temperatures by the processes known from the prior art, which gives rise to thermal damage and degradation products, and this has the disadvantage of worsened optical properties due to defective structures.
Efficient concentration of the polycarbonate solution and vaporization of the residual contents of solvents at low temperatures is therefore of utmost importance for obtaining polycarbonates with improved optical properties.
There have been a variety of literature descriptions of the process for polycarbonate synthesis by the interfacial process, for instance in Schnell, “Chemistry and Physics of Polycarbonates”, Polymer Reviews, Volume 9, Interscience Publishers, New York, London, Sydney 1964, pages 33-70.
In the interfacial process, a disodium salt of a biphenol (or of a mixture of different biphenols), initially charged in aqueous alkaline solution (or suspension), is phosgenated in the presence of an inert organic solvent or solvent mixture which forms a second phase. The oligocarbonates which form and are present principally in the organic phase are condensed with the aid of suitable catalysts to give high molecular weight polycarbonates dissolved in the organic phase. The organic phase is finally removed and washed in a multistage process in order to remove residues of sodium and catalyst. Typically, the organic phase contains, after the reaction, 10-20% by weight of polycarbonate.
The polycarbonate subsequently has to be isolated from the organic phase. The common processes for concentrating the polycarbonate solution and for isolating the polycarbonate are described in the patent literature and in textbooks, and are familiar to those skilled in the art. The isolation of the polycarbonate from the solution is preferably performed by vaporizing the solvent thermally or by means of vacuum. In order to directly obtain the melt phase after the vaporization of the solvent, this process requires the use of a high-boiling (>100° C.) solvent, for example chlorobenzene. In order to improve the solubility of the polymer in the solvent during the reaction, a mixture of one or more high-boiling solvents and the low-boiling dichloromethane is also used. The weight ratio of dichloromethane to the high-boiling solvent is typically about 1:1.
One means of preparing polycarbonate without detectable amounts of residual solvent is the preparation by the transesterification process. This process too is familiar to the person skilled in the art and is likewise described in Schnell, “Chemistry and Physics of Polycarbonates”. In this process, the monomers, a bisphenol or a mixture of different bisphenols, are reacted with a diary! carbonate or a mixture of different diaryl carbonates, in an equilibrium reaction. The by-product formed here is a phenol or a mixture of phenols. These phenols are removed to build up to the desired molecular weight.
Polycarbonates prepared by the transesterification process inevitably contain the phenols formed in the reaction and residues of the bisphenol and diaryl carbonate (for example diphenyl carbonate) monomers. Residual diphenyl carbonate contents are, for example, in the range from 200 to 700 ppm. These substances are likewise disruptive. They are partly released during processing such as injection molding and extrusion at the processing site, and lead to odor nuisance and environmental pollution there. In addition, they can lead to deposit formation in injection molding and hence to reduced service lives. They can also be transferred from the polycarbonate to food and drink on contact with food and drink, and lead to changes in taste therein. Water is particularly sensitive to changes in taste. Particularly phenols tend to form halogenated phenols when food containers made of polycarbonate, in the course of cleaning and/or disinfection, come into contact with chlorine-active agents or strongly oxidizing agents in the presence of chlorine or bromine ions. The taste threshold of phenol in water is specified in the literature at 10 μg/1 (Young & Crane et al., 1996); that of the halogenated phenols is about a factor of 500 lower (H. Burttschel et al., J. Am. Water Works Assoc., 51:205 (1959) “Chlorine derivative of phenol causing taste and odor” and C. Joll et al., Curtin University of Technology, Centre for Applied organic Geochemistry, “The Chemistry of Halophenol Tastes in Perth Drinking Water”). Therefore, residual phenol contents in polycarbonate are particularly unfavorable for water.
In addition, polycarbonates prepared by the transesterification process inevitably contain residues of catalysts. These catalysts are known to those skilled in the art and are disclosed in numerous patent specifications. They may, for example, be alkaline compounds of alkali metals and alkaline earth metals, for example sodium phenoxide, for example in concentrations greater than 30 ppb (based on sodium). Such compounds are unfavorable for the quality and stability of the polycarbonate. Since it is necessary, as is well known, for phenolic OH end groups in the transesterification process to react with aryl end groups with an increase in molecular weight, polycarbonates prepared by the transesterification process inevitably contain a certain minimum content of phenolic OH end groups. Polycarbonates prepared industrially by the transesterification process have concentrations of phenolic OH end groups which are, for example, above 200 ppm. Phenolic OH end groups are particularly harmful for polycarbonates because they adversely affect the stability of the polycarbonate and can lead, for example, directly to the redissociation of phenol and to the reformation of diaryl carbonates. The phosphonium catalysts used, for example, for the preparation of polycarbonate, for example tetraphenylphosphonium phenoxide, have the advantage of decomposing thermally, but likewise remain in the product in small residues and likewise reduce the stability of the polycarbonate.
A further means of preparing polycarbonate consists in the phosgenation of bisphenols in the presence of pyridine or mixtures of pyridine and chlorobenzene, as described, for example, in U.S. Pat. No. 3,114,432. Polycarbonates with residual pyridine contents are entirely unsuitable for food and drink applications due to the intense unpleasant odor.
Halogenated solvents exhibit similarly low sensory thresholds to phenols and the halogenated derivatives thereof. They do possess lower solubilities and migrate more slowly due to the lower diffusion constants thereof, but are converted to water according to the conditions and thus cause changes in taste. In taste tests, test subjects detected changes in taste even at chlorobenzene contents in water of 1 ppb. In order to reliably rule out such a change in taste, a residual chlorobenzene content in drinking water bottles produced from polycarbonate of less than 10 ppm is required.
The thermal degradation of polycarbonates can also give rise to cresols which, as a result of their intense taste, can likewise lead to changes in taste in foods.
A further means of preparing polycarbonate consists in the reaction in the phase interface with subsequent isolation of the polycarbonate from the organic solvent by injection of a heated gas, in particular steam, to drive out the volatile constituents. This involves spraying the polycarbonate solution with the carrier gas, and obtaining polycarbonate as a solid, in particular as a water-moist suspension. Other isolation methods are crystallization and precipitation, and the baking-out of the residues of the solvent in the solid phase. The latter process entails the use of dichloromethane as a solvent, it being possible to achieve residual contents of volatile solvent of about 2 ppm of dichloromethane.
However, residual contents of dichloromethane are particularly disruptive in the polycarbonate since dichloromethane is known to eliminate hydrochloric acid together with residual moisture in the processing operation, and can thus lead to discoloration of the polycarbonate and to corrosion on tools. At elevated temperatures, dichloromethane can also lead to losses of quality such as discoloration and gel formation in the workup operation.
In the case of phosgene preparation from chlorine and carbon monoxide, which is required for the interfacial process, any secondary methane component which occurs is known to be converted to carbon tetrachloride. In the spraying process, the carbon tetrachloride, which is a high boiler, is enriched compared to the dichloromethane, a low boiler, such that residual carbon tetrachloride contents in the region of up to 2 ppm can also remain after the spraying process. Residual carbon tetrachloride contents are, as the person skilled in the art knows, particularly undesirable in the product.
A further method is the isolation of polycarbonate from solution by injecting vapors of aromatic, nonchlorinated aromatic compounds, for example benzene, toluene, ethylbenzene or various xylenes, into a polycarbonate solution in dichloromethane with subsequent solidification and drying, as described, for example, in DE 3 429 960. Residual contents of aromatic compounds can likewise alter taste. No method for reliable removal of carbon tetrachloride and dichloromethane is taught in DE 3 429 960. A considerable disadvantage of this method comes to light in the industrial conversion. For this purpose, for reasons of economic viability and of environmental protection, it is absolutely necessary to close the substance circuits. In particular, the aromatics used, after removal from the polycarbonate, have to be recycled into the process. Low molecular weight constituents of the polycarbonate, for example thermally unstable bisphenols, are vaporized together with the solvent in the course of drying. They are subject to thermal and possibly oxidative stress in the circuit. The person skilled in the art knows that, for example, bisphenols are converted under thermal stress to colored, in particular yellow, compounds. These colored compounds are enriched in the circuit, and so they lead during prolonged operation to a constant deterioration in the colors of the polycarbonate produced. Industrial production of polycarbonates with a light intrinsic color is therefore impossible by this process. This effect does not occur in short-term tests as described in the examples of DE 3 429 960. Carbon tetrachloride is also enriched in this circuit in the course of prolonged operation, which eventually leads to unacceptably high contents of carbon tetrachloride in the polycarbonate.
Residual contents of high-boiling solvents such as aromatic hydrocarbons and chlorohydrocarbons are likewise disruptive. They are partly released during processing such as injection molding and extrusion at the processing site, and lead to odor nuisance and environmental pollution there. In addition, they can lead in injection molding to deposit formation and hence reduced service lives.
They can also be transferred from the polycarbonate to food and drink on contact with food and drink, and lead to changes in taste therein. An adverse effect on taste can already be found at residual contents of above 10 ppm of aromatic chlorohydrocarbons in the polycarbonate. There are no known prior art processes which reduce the residual content of aromatic hydrocarbons, especially chlorohydrocarbons, to a level of between 0.1 ppm and 10 ppm, and at the same time afford a product which reduce less than the detection limit of 0.5 ppm of dichloromethane and less than 15 ppm of phenols, and which at the same time can restrict any carbon tetrachloride which occurs to residual contents of less than the detection limit of 0.01 ppm, and afford constantly good color and thermal stability.
In the known processes for vaporization, or else flash vaporization, polycarbonate solutions are repeatedly heated under slightly elevated pressure to temperatures above the boiling point and these superheated solutions are subsequently decompressed into a vessel, the pressure in the vessel being lower than that corresponding to the vapor pressure in the solution. The repetition of the process is generally favorable since the concentration of polycarbonate in the solution after the reaction is relatively low, and the repetition of the process allows significant overheating to be avoided. Common processes for the evaporation of polycarbonate solutions using apparatus are familiar to those skilled in the art. For example, the superheated solution can be decompressed into a heated helical tube which opens into a separator.
Above a particular concentration of polycarbonate (about 60% by weight), evaporation by flash vaporization becomes more difficult as a result of the high viscosities. Evaporation up to about 60% is referred to hereinafter as preliminary evaporation. It is generally favorable to remove the residual solvent with other processes, apparatuses and machines. These may be, for example, devolatilizing extruders or vertical tubular devolatilizers. At the last stage, it is also possible to use strand devolatilizers or foam devolatilizers in order to achieve particularly low residual contents.
In the evaporation of polycarbonate according to the prior art, usually excessively high apparatus temperatures and excessively long residence times of the melt in the apparatus are employed, with which there is sufficient removal of residual volatile constituents in the polycarbonate melt, but damage to the polycarbonate thus prepared occurs. This product damage is usually a direct consequence of excessive thermal stress over the course of excessive residence time of the polymer melt in the devolatilizing apparatus. In the course of this, side reactions occur on the polycarbonate, which cause a deterioration in the optical properties, especially the formation of defective structures, which usually only become visible in UV light in the moldings produced from such polycarbonate. Examples of such defective structures are ultrafine particles and gel bodies. In the processing of polycarbonate to give optical data carriers, for example CDs or DVDs, such defective structures in the end product cause a considerable loss of quality, which is intolerable and has to be avoided.
EP 1 088 019 discloses the devolatilization of polycarbonate which has been prepared by the interfacial process, by means of a multistage preliminary evaporation with a final strand devolatilizer. An achieved concentration of aromatic chlorohydrocarbons (chlorobenzene) of 20 ppm is described therein.
Concentrations of chlorobenzene-containing polycarbonate solutions are described in EP-A 1 265 944 and EP-A1 113 848, the examples of which describe the preparation of 65% by weight polycarbonate solutions. For removal of residual volatiles in the polycarbonate, in contrast to the subsequent process steps described therein, such polycarbonate solutions can also be evaporated further in devolatilizing extruders.
EP 1 265 944 discloses a process for devolatilizing polycarbonate which has been prepared by the interfacial process with a strand or pipe devolatilizer. The lowest residual contents of aromatic chlorohydrocarbons achieved in the examples are 25 ppm.
EP 1 113 848 likewise discloses a process for devolatilizing polycarbonate which has been prepared by the interfacial process with a strand or pipe devolatilizer as the last stage. The lowest residual content of aromatic chlorohydrocarbons achieved in the examples are 50 ppm.
Such removals of residual volatiles from polycarbonate solutions with the aid of devolatilizing extruders are described in DE 29 08 352 and EP 1 165 302. In these two extruder processes, what is called backward devolatilization at the extruder inlet is described. In this case, an optionally preheated polymer solution is introduced into a twin-screw extruder and foams therein. The gases are then removed backward through the flights of the twin-screw extruder to a devolatilizing dome. In general terms, such backward devolatilization is prior art and is described, for example, in the textbook “Der gleichläufige Doppelschneckenextruder” [The Corotatory Twin-Screw Extruder], Klemens Kohlgrüber, Carl Hanser Verlag, ISBN 978-3-446-41252-1 [1], on pages 193-195. One disadvantage of backward devolatilization is a limitation in the amount of solvent evaporated off because the screw channels are relatively narrow and, as a result, high gas velocities are achieved, which can lead to entrainment of product into the backward devolatilizing dome. Thus, a relatively high proportion of residual solvent has to be evaporated out in the further stages of the extruder if 65 to 75% by weight polycarbonate solutions are introduced into these apparatuses and are to be concentrated down to a few ppm, based on the overall polycarbonate material, of residual solvent contents in the polycarbonate. Thermal damage to the polycarbonate may occur, for example yellowing, formation of insoluble constituents, specks, cleavage of the polymer chains, formation of residual monomers and other low molecular weight constituents, etc. It is also disadvantageous when a polycarbonate solution with residual contents of solvent, such as dichloromethane, is fed directly into an extruder, since the overheating of the solution, which is well known to the person skilled in the art, on the flights of the screw in the presence of, for example, dichloromethane can lead to local product damage and hence to discoloration of the overall product. The speeds of up to 390/min specified in EP 1 165 302 are absolutely necessary for good devolatilization, but lead at the same time to a significant temperature rise in the product and therefore to discoloration and formation of low molecular weight components in the polycarbonate.
One method for residual devolatilization of polycarbonate solutions with the aid of foaming agents is WO 2005/103114. This describes the vaporization of the organic solvent up to a polycarbonate content of 90 to 99.95% by weight, optional mixing of the melt thus obtained with a foaming agent and the devolatilization of the from one of the preceding melt by introduction via entry orifices into a separation vessel under reduced pressure. The residual contents specified in the examples were a minimum of 7 ppm of aromatic chlorohydrocarbons (chlorobenzene). The conducting elements specified therein for the melt under reduced pressure, which lead to particularly low residual contents, are unfavorable due to the risk of formation of gel.
One method for reducing residual contents of monomers and phenols in polycarbonates which have been prepared by the transesterification process is described in EP 1 742 983. This process achieves a reduction in the residual contents down to about 30 ppm of diaryl carbonate (in this case diphenyl carbonate). The conductive elements specified therein for the melt under reduced pressure, which lead to particularly low residual contents, are unfavorable due to the risk of formation of gel. This method too, owing to the reactivity of the polycarbonate prepared by transesterification, is unsuitable for reducing the residual phenol contents to less than 15 ppm.
Further prior art for removal of residual volatiles from solutions of thermoplastic polymers is EP 1 556 418, in which the injection and distribution of liquids, especially water, under pressure through specially configured liquid distributors in the polymer melt stream is described. hi the course of decompression in a separation chamber, with the aid of the vaporizing liquid, foaming of the polymer melt and hence concentration by vaporization of the solvents is achieved. The use of water as a foaming agent for the removal of residual volatiles from polycarbonate melts is inadvisable due to the risk of polymer degradation by hydrolysis. The European patent, incidentally, does not give sufficient teaching as to the configuration of the apparatus in which the devolatilization and concentration take place.
Further prior art for removal of residual volatiles from solutions of thermoplastic polymers is EP 905 149, in which the injection of blowing agents into the polymer stream counter to the flow direction thereof and the distribution of blowing agents, for example water or volatile aliphatic hydrocarbons, in this polymer stream are described. The use of water as a foaming agent for the removal of residual volatiles from polycarbonate melts is inadvisable due to the risk of polymer degradation by hydrolysis, and the use of other solvents is inappropriate with regard to recoveries in circuits and the remaining residual contents in the product.
In addition, EP-A 027 700 discloses the combination of a flash devolatilizer with a devolatilizing extruder for concentration of the solutions from olefin polymerization, the flash stage being preceded by injection of stream as an entraining agent into the polymer melt stream. In the case of polycarbonate melts, water at elevated temperature can lead to polymer degradation by hydrolysis.
Therefore, such a process is inadvisable for the removal of residual volatiles from polycarbonate melts. It is also stated therein that the product is “collected” in the devolatilizing vessel in the base of the apparatus, and is supplied to the extruder in contact with the base of the devolatilizing vessel, which leads to increased residence times of the polymer and hence to thermal damage.
EP 1 113 848 B1 described, for the last stage of the evaporation, a combination of pipe and strand devolatilizers. This involves first concentrating the polymer solution in a vertical shell-and-tube heat exchanger with downstream separator, proceeding from a solution containing 60% to 75% by weight of polycarbonate, to 98 to 99% by weight, and then concentrating it in a strand devolatilizer to residual contents of 5 to 500 ppm of chlorobenzene. In the case of use of a strand devolatilizer, the polymer melt is shaped to fine strands in a separator under reduced pressure and elevated temperature and thus freed of the solvent. The disadvantage of the strand devolatilizer technique is that effective devolatilization is ensured only by stable strands, meaning that they do not tear in the apparatus. The stability of the strands is influenced by the viscosity of the polymer solution. Too low a viscosity can lead to strand breaks. This leads to a restriction in the operating parameters with regard to temperature and inlet content of residual volatiles. In addition to the adverse influence on the viscosity, an excessive inlet concentration of volatiles directly has adverse effects on the success with which devolatilization can be achieved, since the mass transfer is determined purely by diffusion. The surface area for the mass transfer is, in contrast, fixed by the strand geometry. The requirement for a large area of the melt distributor required to obtain the strands additionally entails expensive, large apparatuses. These large apparatuses in turn inevitably have large areas which, in particular in the discharge, are flowed through by low flow rates. Such low flow rates lead to excessively long residence times of the polycarbonate close to the walls and induce unwanted changes in the polycarbonate there, such as discoloration and formation of gel.
[1], p. 193, FIG. 10.1 specifies, in schematic form, two processes for devolatilization of monomers or small amounts of solvent.
One process described therein (middle line of the figure) consists schematically of an addition zone, a kneading zone, a entraining agent addition in a mixing zone, a devolatilizing zone, a further entraining agent addition in a mixing zone, a further devolatilizing zone and a subsequent granulation. The kneading zone in this process leads to a high energy input, which is detrimental to product quality. Since only two devolatilizing zones are envisaged, devolatilization is not very successful. A further increase in the number of devolatilizing zones in this arrangement is impossible because the thermal stress would rise to too high a level as a result.
A further process described therein (lower line of the figure) consists schematically of an addition zone, a kneading zone, a devolatilizing zone, an entraining agent addition in a mixing zone, a further devolatilizing zone and a subsequent pelletization. The kneading zone in this process leads to a high energy input, which is detrimental to product quality. The first devolatilizing zone does not perform at low inlet concentrations as a result of the absence of entraining agent, and so the overall degassing performance of this schematic configuration is even lower than that of the middle line.